Portable transmission system



July 2, 1940.

E. Y. WEBB, JR

PORTABLE TRANSMISSION SYSTEM Filld Nov. 18, 1939 2 Sheets-Sheet 1 awe/WM Y. mama July 2, 1.

E. Y. WEBB, JR

PORTABLE TRANSMISSION SYSTEM 2 Sheets-Sheet 2 Filed Nov. 18, 1939 w A m Patented July 2, 1940 UNITED STATES 2,206,590 PORTABLE ramsmssrou svs'rnm Y. Webb, J12, Atlanta, Ga.

Application November 18, 1939, Serial No. 305.1 45

This invention relates generally to communication systems, and more particularly to portable, self-contained privacy units adapted to be detachably connected to separated terminals of any existing wire or radio voice circuits, for use. alternatively, as a means for inverting the speech band for transmission in inverted form from one terminal to the other, or as a means for receiving a similarly inverted speech band to reinvert the same and thereby reproduce the original speech band.

The main object of the invention is to provide a portable, self-contained privacy unit which can be readily attached to any communication circuit without requiring any special treatment of that circuit, or interfering with its normal operation. This feature renders the invention particularly advantageous for use on telephone circuits by business houses, Government officials, Army field use, naval service or any agent desiring privacy in the transmission of audible messages between terminals of a voice circuit.

Another object of the invention is to provide a self-contained privacy unit of the character described which is reversible in operation, so as to adapt it not only for inversion of a speech band for transmission in inverted form to a similar unit 9 similar unit at a distance to re-invert the same and thereby reproduce the original speech band.

A further object of the invention is to provide a privacy unit of this character with means for by-passing the unit from the communication circuit and for changing it instantly from sending to receiving condition, and vice versa.

Other objects of the invention will become apparent as the detailed description thereof proceeds.

In the drawings:

Fig. 1 is a perspective view of one of the privacy units embodying the present invention and adapted to be connected, for both sending and receiving purposes, to a telephone circuit; and

Fig. 2 is a schematic diagram ,of one of said units, illustrated as applied to one terminal of a telephone circuit.

Referring now to Fig. 2, ordinary speech, entering the line L associated with the usual type telephone transmitter and receiver, is impressed on the low pass filter F, through the auto-transformer AT. This simple, balanced filter, of well known design, eliminates frequencies higher than those required for normal speech' transmission and prevents themfrom appearing as distortion in the output circuit of 'the unit. This selected band of frequencies is then impressed across a bilateral oxide modulator M, to which is also applied a modulating or carrier frequency from a generator G. For complete privacy, it is essential that the original input speech shall not pass into the output circuit. This result is secured,

with high precision, by adjustingthe balance of the two modulator branches by means of the variable potentiometer P.

The modulator M produces, in a manner wellknown, certain frequencies which are combina tions of the voice frequencies and the modulating frequency of the generator G. Among these frequencies are the upper and lower side bands. commonly referred to as C'+V and C'-V, which are, respectively, the carrier frequency plus the voice frequencies, and the carrier frequency minus the voice frequencies. It will be obvious that the upper side band C+V is the original voice frequencies raised in pitch by the value of the carrier frequency of the generator G. It will also be obvious that the lower side band C-V is the inversion of the original input frequencies, so that low input frequencies appear as high, and high input frequencies appear as low. The lower side band, being inverted, is unintelligible until it is restored to its original form by a similar process or modulation.

The filter F-l, similar in construction to the filter F, eliminates the upper side band C+V and higher frequency components, but transmits the range of frequencies of the lower side band -4. To make ordinary detection of the inverted side band ineffective for reproducing intelligible speech, the modulating or carrier frequency impressed on the modulator M does not appear in the output circuit, which therefore requires that a similar carrier frequency be supplied at the receiver terminal of the system.

The carrier frequency from the generator G is balanced out of the filter and output circuit by means of the potentiometer P--2, which permits a high degree of balance, so that the electrical center of the filter and output circuit can be obtained to secure exactly equal potentials across the filter terminals. 'By' this construction, the attenuation requirements of the low pass filter F-l are reduced to a minimum, with a corresponding reduction in the size and weight of its elements, since this filter is not required to eliminate the carrier frequency, the elimination of which would otherwise require a large attenuation in this frequency range. This method of balancing obviates the use of center tapped transformers or any transformers. The variable potentiometer P-l permits the carrier fiequency from the generator G to be applied to the exact electrical center of the input terminals of the modulator M, thus balancing out all the even harmonics of the carrier frequency, being generated in the modulator.- The side band (2-1- from the filter F-l, is appliedthrough the transrdrmer'rtotneamplifyinembemandtnenca line 'I'L.

The directionof transmission through the'unit, is determined through the operation of a push button switch B which controls the operation of the relay R. This construction eliminates the necessity of using the balanced bridge coil, in

common use in this art, and thereby greatly.

reduces the size and weight of the unit, as well as. reducing the amount of amplification required necting it from, the telephone or voice circuit. A

complete unit of this privary system is shown in Fig. 1 to emphasize, by comparison with the ordinary telephone transmitter-receiver, the portability of the system, the privacy of which may be enhanced by periodically changing the values of the generator condenser to secure different secrecy combinations.

"I'wo structurally identical units, one for each end of the communication circuit, form this pri-' vacy system. The units are self-contained, and are connected directly to any communication circuit without additional equipment. For use on a telephone circuit, the two leads from each unit are connected in series with the circuit. When this connection is made, each unit may be connected to, or disconnected from, the circuit by operation of the-- switch S. In the disconnect position of the switch, communication may be. carried on in the normal manner. When the switch S is in theconnect position at each terminal, each unit is in the receive condition. order to talk, the push button B on the control .box CB must be pressed to change the circuit from receiving to transmitting condition. Release of the button '3 restores the circuit immediately to receiving condition. When the unit is connected to a radio telephone circuit, one of the leads provided with the unit connects the input to the transmitter, and the other lead connects to the receiver output circuit, and the operation is the same as for the telephone circuit.

What I claim is:

1. The method of secret telephony which comprises modulating an audio frequency carrier wave in accordance with speech, selecting from the modulated waves the lower side band comprising inverse frequency speech waves, balanc ing the carrier frequency out of said side band, transmitting the resultant inverse frequency speech waves to adistant point, receiving the transmitted waves and 'reinverting the frequencies thereof to reproduce the original speech.

' and reinverting the frequencies ,duce the original speech.

said side band, balancing out the carrier frequency from the side band, transmitting the resultant inverse frequency speech waves to a' .distant point, receiving the transmitted waves thereof to repro- ,,3, The method of secret telephony which 3,208,590 through the transformer 'I-.-l to the transmission prises eliminating from a speech frequency wave all frequencies higher than those required for normal speech transmission, modulating the resultant band of frequencies along with a modulating carrier frequency, balancing out the harmonics of' the carrier frequency, balancing out the carrier frequency, amplifying and transmitting the resulting inverse frequency speech waves to a distant point, receiving the transmitted waves and applying thereto in reverse order the stepsapplied prior to transmission to reinvert the frequencies of said waves and thereby reproduce the original speech wave.

4. In asecret signaling system, a privacy unit adapted to be detachably connected to a transmission line and comprising, in series, a transformer having its secondary tapped for operation as an auto-transformer, a low-pass filter, an oxide modulator having means for adjusting the balance of its branches, a generator, a potentiometer adjustable to apply a modulating carrier frequency from the generator to the electrical center .of the modulator input terminals, a second lowpass filter, means for balancing the carrier frequency out of the second filter, means for amplifying and transmitting the output of the second filter to the transmission line, and a second privacy unit structurally identical with the first unit and connected to the transmission line to receive the output of the first unit, each unit being, provided with means for alternatively changing it from sending to receiving condition, the primary of the transformer being operable only during the receiving condition of the unit,- and the secondary operating only as an auto transformer when the unit is in sending'condition.

5. The system set forth in claim 4 in combination with means for by-passing either unit from the transmission line.

6. The s stem set forth in claim 4, in which said means cludes a manually operable switch for reversing the. direction of transmission through the units.

7. In a secret signaling system, a privacy unit adapted to be detachably connected to a transmission line, and comprising means for alternatively trannforming intelligible speech into an unintelligible speech band for transmission on said line, or for receiving and retransforming into intelligible speech a speech band rendered unintelligible by a similar unit, and means aijustable to adapt said unit, alternatively, for ransmission or reception of suchunintelligible speech band.

8. The system set forth in claim 4; in which said means includes a manually operable switch 9. In a secret signaling system, a privacy unitadapted to be detachably connected to a transmission line and comprising, in series, a transformer, a low pass filter, an oxide modulator having means for adjusting the balance of its branches, a generator, a potentiometer adjustable to apply a modulating carrier frequency from the generator to the electrical center of the modulator input terminals, a second low pass filter, means for balancing the carrier frequency out of the second filter, and means for amplifying and transmitting the output of the second filter to the transmission line.

EDWDI Y. WEBB, JR. 

